After seeing this stuff, I have some theories about the bottom markings:

The first letter denotes the fab, the second letter I am not too sure about, while the next two numbers are the week the dice were made. the remaining 4 letters are a wafer lot code most likely. Some chips have a number after this which appears to be the die revision, but only from late 84 on appears to have it. The 4684 and 2285 date coded chips have a 2 and 3, respectively which appears to be the die revision.

Later chips were actually marked R3 (at the tail end of production) and all R4 and on chips were marked. The only exception I can find is my 6582 chip which has to be an R5 no matter what.